United and Arsenal can't just go mad because they are spending their own generated money, so there's a natural limit there - go beyond that and they'd need to take on debt which will eventually have to be paid back. City, Chelsea and PSG have no such limit as it's their owners' money being spent, and they are all multi-billionaires playing games in a sport where even the biggest real clubs don't spend more than ~£150m in a summer.
FFP failed when UEFA accepted the sponsorship shams that City and PSG invented to make their figures look better - if UEFA had taken a strong hand with them then I'd guess the FFP position of both clubs would be very different.